1. Field of the Invention
The present invention relates to a cell search method and apparatus for a mobile station in a mobile communication system.
2. Description of the Related Art
In a mobile communication system based on the CDMA (Code Division Multiple Access) method, if a mobile station communicates with a base station or measures power received from a base station, it must detect frame boundaries and a scramble code in a down signal from the base station. This is called “cell search”.
The cell search method essentially comprises descrambling the signal at all possible scramble codes with all possible timings and selecting a timing and a scramble code with which a correlation coefficient obtained as a result of despreading with spreading code used is largest, thereby detecting frame boundaries and the scramble code for the base station. This method, however, requires a large amount of time for the cell search. To increase the speed of the cell search, a method in which the base station transmits each slot through a PSCH (Primary Synchronization CHannel) and an SSCH (Secondary Synchronization CHannel) is used (refer to 3GPP Technical Specification 25.211).
FIG. 1 shows a configuration of a down channel relating to the cell search. This down channel relates to the cell search in the W-CDMA method (refer to 3GPP Technical Specification 25.211), which is representative of the CDMA method. On a primary synchronization channel, a spreading code PSC (Primary Synchronization Code) is used, which is common to all cells and slots, and the signals is transmitted in accordance with slot cycles. On a secondary synchronization channel, different spreading codes SSC0 to SSCn−1 (Secondary Synchronization Codes) are used for the respective slots, one frame constitutes a spreading code sequence, and is repeatedly and cyclically transmitted. Different spreading code sequences are used for respective cells, and are each correlated with a scramble code group to which the scramble code used by the base station belongs. The primary and secondary synchronization channels are not subjected to scramble codes. On a common pilot channel (CPICH), the signals is transmitted by using a spreading code and a symbol pattern which are common to all the cells and subjecting the respective cells to different scramble codes.
FIG. 2 shows a first step operation performed in a conventional cell search method. On the primary synchronization channel, the spreading code PSC, which is common to all the cell and slots, is used. A mobile station inputs a received signal to a matched filter 201 corresponding to this spreading code PSC, and causes an multi-slot averaging process section 202 to execute averaging over a plurality of slots in order to reduce the adverse effects of noise and interference. A peak detector 203 selects a timing with which an average correlation coefficient is largest, to detect slot boundaries. In the W-CDMA method, one slot corresponds to 2,560 chips, so that there are 2,560 candidate timings for the slot boundaries if no oversampling is executed. The above described operation is called a “first step”.
On the secondary synchronization channel, the different spreading codes are used for the respective slots, and one frame constitutes a spreading code sequence. The spreading code sequence is repeated in accordance with frame cycles, and different spreading code sequences are used for the respective cells. These spreading code sequences are correlated with respective groups of scramble codes so as to allow the scramble codes to be subsequently detected easily. Since the slot boundaries have been detected at the first step, the mobile station can calculate a transmission timing on the secondary synchronization channel.
The mobile station then despreads a received signal using the calculated timing and the spreading codes SSC for the secondary synchronization channel, averages correlation output coefficients corresponding to all possible frame boundaries and SSC sequences, and selects a timing and an SSC spreading code sequence with which the average correlation coefficient is largest. The mobile station thus detects the frame boundaries and a scramble code group. This operation is called a “second step”.
FIG. 3 shows a second step operation performed in the conventional cell search method. A timing with which transmissions are executed on the secondary synchronization channel are calculated on the basis of the slot boundaries detected at the first step. Correlators 3011 to 30116 corresponding to the secondary synchronization codes detect a correlation using the calculated timing. The W-CDMA method involves 16 secondary synchronization codes. Thus, the correlation operation is performed over a plurality of slots. Since the secondary synchronization code pattern is repeated on the secondary synchronization channel so as to correspond to the frame cycles, averaging process sections 3021 to 30216 execute an averaging process over the frames as required. In the W-CDMA method, a maximum of 16 secondary synchronization codes are used for each slot to achieve the correlation, and one frame consists of 15 slots. Accordingly, after the inter-frame averaging, a maximum of 16×15 average correlation coefficients are output and stored in a correlation value memory 304. Subsequently, a C2k1 calculating section 305 averages the correlation coefficients so as to correspond to timings and SSC spreading code sequences that are possible within the frame. In the W-CDMA method, if the slot boundaries are known, 15 timings are possible for the frame boundaries and there are 64 possible SSC sequences, so that a maximum of 15×64 average correlation coefficients are calculated. As a result, a peak detector 303 detects frame boundaries and a scramble code group by selecting a timing and an SSC spreading code sequence with which the average correlation coefficient is largest.
The mobile station, which has detected the frame boundaries and the scramble code group during the second step, finally receives a signal subjected to a scramble code, through a common pilot channel, and determines which of the scramble codes of the scramble code group equals that of this signal. Since the frame boundaries have already been detected, the phase of the scramble code can be calculated. Since the spreading code for the common pilot channel is common to all the cells, essentially all the scramble codes within the group may be used to descramble the signal, and the spreading code for the common pilot channel may then used to despread the signal. Subsequently, these operations may be performed over a plurality of symbols with the results averaged, and a scramble code may then be selected with which the average correlation coefficient is largest. This operation is called a “third step”.
FIG. 4 shows a third step operation performed in the conventional cell search method. For example, in the W-CDMA method, one scramble code group contains eight scramble codes. The mobile station calculates the phases of scramble codes on the basis of the frame boundaries detected during the first and second steps, and causes descramblers 4041 to 4048 to descramble the received signal with eight scramble codes. Then, correlators 4011 to 4018 calculate the phases of the scramble codes on the basis of the frame boundaries detected at the second step, and despreads the signal using the spreading code for the common pilot signal. The mobile station performs this operation over a plurality of symbols, and causes averaging process sections 4021 to 4028 to average the results. A peak detector 403 then selects a scramble code with which the average correlation coefficient is largest, to detect a down scramble code used by the base station.
FIG. 5 shows a conventional method of judging the detected frame boundaries and scramble code. If the first to third steps are defined as one search, then after one search has been completed, the mobile station causes a descrambler 504 to descramble the received signal on the basis of the detected frame boundaries and scramble code. Then, a correlator 501 despreads the signal with the spreading code for the common pilot channel, and a pilot symbol demodulating and error measuring section 502 demodulates a pilot symbol on the common pilot channel and measures the number of errors in the pilot symbol. A detection result judging section 503 then executes threshold judgements or the like to determine whether or not the correct frame boundaries and scramble code have been detected.
For example, if a control signal from the base station provides the mobile station with information on scramble codes and frame boundaries for surrounding cells or the mobile station has information on scramble codes and frame boundaries for surrounding cells immediately after the last search, the above described three-step search method may be omitted. That is, the cell search can be achieved by assuming a plurality of frame boundaries before and after an expected frame boundary, descrambling the signal with a candidate scramble code in accordance with respective timings, despreading the signal with the spreading code for the common pilot channel, and then selecting frame boundaries and a scramble code with which the average correlation coefficient is largest.
With this method, the cell search can be simplified to reduce search time and power consumption compared to the three-step cell search. Even in this method, after the frame boundaries and scramble code providing the largest average correlation coefficient have been selected, whether or not these frame boundaries and scramble code are correct is determined by descrambling and despreading the received signal again on the basis of the detected frame boundaries and scramble code, decoding the pilot symbol, and measuring the number of errors in this symbol, or using other methods.
The shorter the cell search time of the mobile station is, the lower its power consumption is. Further, handover can be executed at a high speed, thereby enabling smooth communication. Accordingly, that the time required to determine whether the detected frame boundaries and scramble code are correct is desirably short, and this judgement is desirably accurate enough to avoid selecting incorrect frame boundaries or scramble code or unnecessarily repeating the cell search.
With the conventional method, however, after the frame boundaries and the scramble code have been selected, these detection results are judged again, thus requiring correspondingly much time for the cell search. Another problem with this method is that the judgement is not sufficiently accurate if the signal noise interference power ratio is small.